PPAs are normally only for the Linux Mint Main Edition, the one based on Ubuntu, and not for LMDE. Unless you know what you were doing and are sure you can use this PPA on LMDE, I suggest you remove it. Opening the URLs you shared in a browser will indeed give a 404. As you can see if you go up a few steps in the URL path you shared, the only distributions supported are Ubuntu releases (the "debian" from your configuration is not there, hence the 404): http://ppa.launchpad.net/nilarimogard/w ... ntu/dists/